The present invention relates to facilitating wireless communication between two transceivers. More particularly, and not by way of limitation, the present invention is directed to a system and method to modify a Time Division Duplex (TDD) communication mode supported by current Third Generation Partnership Project (3GPP) standards to facilitate wireless communication between pico and macro base stations, and also between two devices operating in license-exempt bands.
The usage of mobile communication, especially over cellular networks, has shown a significant increase during recent years. In parallel to this, there is an ongoing evolution of Third Generation (3G) and Fourth Generation (4G) cellular networks like High Speed Packet Access (HSPA), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX), etc., to support ever-increasing performance with regards to capacity, peak bit rates and coverage.
It is expected that the 3GPP LTE standard will play a major part in the evolution of wireless networks. Other wireless networks such as WiMAX also will be used in some market segments. Both LTE and WiMAX are currently specified for use in licensed bands and mainly for serving mobile terminals or User Equipments (UE's). However, these LTE and WiMAX standards are being considered for other use cases in the future, e.g., for wireless backhaul communication between a pico (or femto) base station and a macro base station. Similarly, they are also being considered for use in license-exempt bands. Aspects of these standards—such as synchronization sequences and presence of pilot symbols—have been designed to deal with mobility and specific UE-oriented scenarios. However, these aspects are not always well suited for the future use cases (e.g., backhaul communication between pico and macro base stations, or communication between two devices operating in a license-exempt band) being considered. While the LTE specification is evolving and some modifications may be made to the standard to handle these use cases better, it is of significant interest to be able to use LTE and WiMAX as currently defined to the extent possible. Such usage of existing standard in its current form for different applications (e.g., backhaul communication between pico and macro base stations, or communication between two devices operating in a license-exempt band) provides economies of scale and allows products and implementations with lower cost.
Both LTE and WiMAX implement Time Division Duplex (TDD) and Frequency Division Duplex (FDD) modes simultaneously. FIG. 1 shows an LTE radio frame 10 in the TDD mode. The LTE radio frame duration is 10 ms. For TDD mode, the frame consists of two 5 ms half-frames 12-13, each half-frame consisting of five sub-frames 15 as shown in FIG. 1. Each subframe 15 is of 1 ms duration and can be allocated to downlink (DL), uplink (UL) or as a special subframe which consists of the Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS) fields. There are two such special subframes shown in the radio frame 10 in FIG. 1. The GP field in the special subframe enables switching between downlink and uplink transmissions.
FIG. 2 is a table 18 showing uplink-downlink allocations for various sub-frames in a TDD-LTE radio frame (e.g., the radio frame 10 in FIG. 1). In the table 18, the letter “D” refers to a downlink subframe, the letter “U” refers to an uplink subframe, and the letter “S” refers to a special subframe. The periodicity of the switch-point between downlink and uplink can be 5 ms or 10 ms. As mentioned before, the switching from downlink to uplink transmission, and vice versa, may be accomplished using the special subframe “S.” In case of FIG. 1, the radio frame configuration 10 can be a type-2 (from table 18 in FIG. 2) frame structure having 5 ms switch-point periodicity and “DSUDDDSUDD” subframe configuration. Thus, as shown in the table 18, sub-frame allocations can be made in multiple ways in an LTE-TDD radio frame. Various versions of WiMAX standard also have similar TDD frame structures with a 5 ms periodicity.
FIG. 3 illustrates a downlink sub-frame 20 in an LTE radio frame (e.g., the radio frame 10 in FIG. 1). Thus, one or more sub-frames 15 (depending on frame structure selected from the available configurations shown in the table 18 in FIG. 2) in the radio frame 10 in FIG. 1 can have the downlink configuration 20. As shown in FIG. 3, a downlink LTE sub-frame 20 includes a number of resource blocks 22 (in the frequency domain) that can be used to carry different types of information—data 23, control signaling 24, or reference symbols 25. A downlink LTE subframe has a minimum set of reference symbols and control signaling that is required. The reference symbols 25 in the downlink subframe 20 are sent even when there is no data to be sent. In addition to the reference symbols 25 shown in FIG. 3, there are synchronization sequences (e.g., Primary Synchronization Sequence (PSS), and Secondary Synchronization Sequence (SSS)) (not shown) sent once every 5 ms. These sequences (not shown) occupy only a part of the bandwidth (6 resource blocks) at the center of the carrier bandwidth and are also sent independent of any user data transmissions in the subframe.
FIG. 4 illustrates an uplink sub-frame 26 in an LTE radio frame (e.g., the radio frame 10 in FIG. 1). The corresponding uplink transmission may be a Physical Uplink Shared Channel (PUSCH) transmission in LTE. Like the downlink sub-frame 20, the LTE uplink subframe 26 also has reference symbols 27 and data blocks 28. However, in case of the uplink subframe 26, the reference symbols 27 are only transmitted when there is corresponding data to be sent.